Solid high-energy compositions, such as propellants, explosives, gasifiers, or the like, comprise solid particulates, such as fuel particulates and oxidizer particulates, dispersed and immobilized throughout a binder matrix comprising an elastomeric polymer.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526, there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the "pot life". Disposal of a cast, cross-linked propellant composition is difficult, except by burning, which poses environmental problems. Furthermore, current state-of-the-art propellant formulations have serious problems that include, but are not limited to, use of nonenergetic binders, high end-of-mix viscosities, thermally labile urethane linkages, and extreme vulnerability to unscheduled detonation.
In view of inherent disadvantages of cross-linked elastomeric polymers as binder matrices, there has been considerable interest in developing thermoplastic elastomers suitable as binder matrices for solid, high-energy composition. However, many thermoplastic elastomers fail to meet various requirements for propellant formulations, particularly the requirement of being processible below about 120.degree. C., it being desirable that a thermoplastic polymer for use as a binder in a high-energy system have a melting temperature of between about 70.degree. C. and about 120.degree. C. Many thermoplastic elastomers exhibit high melt viscosities which preclude high solids loading and many show considerable creep and/or shrinkage after processing.
The present invention is directed to high-energy compositions having binder systems that are based upon telechelic polymers that exhibit thermoplastic elastomeric characteristics.
U.S. Pat. No. 3,870,841, the teachings of which are incorporated herein by reference, is directed to polystyrene which is randomly sulfonated. U.S. Pat. Nos. 3,642,728 and 3,836,511, the teachings of which are also incorporated herein by reference, are directed to the sulfonation of ethylene-propylene-diene rubber (EPDM) at the diene-derived unsaturations which are present at random points along the polymer chain. The ionic sulfate moieties tend to aggregate at lower temperatures, which aggregation is destroyed at higher temperatures, whereby these polymers exhibit thermoplastic characteristics. The processes described in these patents yield polymer molecules consisting of two non-functional hydrocarbon chain ends, and one or more elastically effective inner segments which are bounded by the ionic moieties. The topology of the network derived from sulfonate aggregation, regardless of the average number of ion pairs which participate in an aggregate, is such that no covalent branch points exist, and each primary polymer chain contributes two dangling, nonload-bearing ends. If a polymer chain is sulfonated at only one point, it contains no inner segments and cannot participate in the load-bearing function. Because of these limitations, the molecular weight of the primary polymer chains, and the level of sulfonation, must be sufficiently high to develop adequate toughness and strength, and this results in undesirably high melt viscosities.
Star-branched, low molecular weight, telechelic ionomers have been shown to make excellent thermoplastic elastomers with very low melt viscosities; J. P. Kennedy, et al., ACS Org. Coat. Appl. Polym. Sci. Pro., 46 182 (1982), Y. Mohajer, et al., Polym. Bull., 8, 47 (1983), and S. Bagrodia, et al., J. Appl. Polym. Sci., 29 (10), 3065 (1984). These works deal with polyisobutylene (PIB)-based ionomers. The covalent branch point of the star dramatically increases ionomer network connectivity. Because the ionic groups are placed only at the chain ends, the network is free of non-load-bearing chain ends.
Additional thermoplastic ionomers are described in a U.S. patent application of Robson F. Storey and Scott E. George entitled "Star-Branched Thermoplastic Ionomers" filed on Jan. 6, 1989 as Ser. No. 07/294,320now allowed the teachings of which are incorporated herein by reference.
Ionomers have important potential advantages relative to other types of thermoplastic elastomers for use as binders in high-energy formulations. In particular, ionomers provide much better mechanical stability relative to conventional thermoplastic elastomers which generally have alternating amorphous and crystalline blocks, the amorphous blocks providing elasticity and the crystalline blocks of different polymer molecules forming a physical interlock to give structure to the elastomer. Conventional thermoplastic elastomers have been shown to be effective propellant binders in small scale rocket motors. However, there appears to be a limit to the size of rocket motors which can be constructed with propellants based on conventional thermoplastic elastomers. The physical interlock provided by the crystalline blocks of conventional thermoplastic elastomers tend to give way when subjected to the compressive stress of the great weight of large rocket motors, causing the polymer molecules to slide relative to each other and thereby allowing the propellant to flow and distort. Ionomers, in contrast, rely not on a mechanical interlock, but on ionic interaction between polymer molecules. This interaction is much stronger than the mechanical interlock in conventional thermoplastic elastomers; thus, ionomers hold the promise of being much more suitable than conventional thermoplastic elastomers for large rocket motors. At the same time, ionomers exhibit thermoplastic, elastomeric characteristic, providing advantages inherent in conventional thermoplastic elastomers relative to cross-linked elastomers.
Despite the recognized potential of ionomers as the basis for binder systems in high-energy compositions, attempts to produce ionomer-based high-energy formulations in a practical manner have been limited by processing difficulties. In particular, ionomer-based propellant formulations have proven to be too viscous to process in conventional mixing apparatus, such as sigma blade mixers, or in conventional extrusion apparatus.
It is a general object of the invention to provide high-energy formulations having solid particulates in an ionomer-based binder system, which high-energy formulations are castable and extrudable.